1. Field of the Invention
The present invention relates to imaging systems and, in particular, to imaging systems which increase photodetector charge capacity towards the end of the integration period to extend the dynamic range of the imaging system.
2. Description of the Related Art
Various types of imagers or image sensors are in use today, including charge-coupled device (CCD) image sensors and complementary metal-oxide semiconductor CMOS image sensors. These devices are typically incorporated into CCD and CMOS imaging systems, respectively. Such systems comprise an array of pixels, each of which contains a light-sensitive sensor element such as a CCD or, in CMOS image sensors, a virtual gate buried n-channel photodetector, a N+ to p-substrate photodiode, or a photogate detector. Such light-sensitive sensor elements will be referred to herein, generally, as photodetectors.
In such devices, the photodetector accumulates charge and hence voltage during the optical integration period in accordance with the light intensity reaching the relevant sensing area of the photodetector. As charge accumulates, the photodetector begins to fill. The charge stored in a photodetector is sometimes said to be stored in the "charge well" of CCD-type photodetectors. If the photodetector becomes full of charge, then excess charge is shunted off, in part to prevent blooming. Blooming is a phenomenon in which excess charge beyond pixel saturation spills over into adjacent pixels, causing blurring and related image artifacts. However, if the photodetector becomes full before the end of the integration period and any additional photons strike the photodetector, then no additional charge can be accumulated. Thus, for example, if very bright light is applied to a photodetector, this can cause the photodetector to be full before the end of the integration period and thus to saturate and lose information.
U.S. Pat. No. 3,953,733, issued Apr. 27, 1976 to Levine ("Levine"), the entirety of which is incorporated herein by reference, teaches a method of operating CCD imagers to avoid this problem. The voltage applied to the electrodes of a CCD cause a heavily depleted region to form beneath the electrode, which forms "potential wells" or charge wells of a given maximum charge capacity. A greater electrode voltage causes a correspondingly greater charge capacity well to form. The voltage that controls the maximum charge capacity of a photodetector, such as the CCD electrode voltage, will be referred to herein as the charge capacity control voltage, and the maximum charge that can be accumulated in a photodetector will be referred to herein as the photodetector's charge capacity. The charge capacity control voltage is also sometimes referred to as the blooming barrier voltage, since it acts as a blooming drain to remove charge from the pixel photodiode to avoid charge spilling into adjacent pixels during optical overload.
Typically, the charge capacity control voltage applied is constant throughout the integration period, so that a given charge capacity exists throughout the integration period for each pixel of the imager array. In Levine, the charge capacity control voltage is varied during the integration period, so as to increase the optical dynamic range of the CCD imager. For example, in one embodiment, Levine teaches increasing the charge capacity control voltage (and hence the charge capacity) in non-linear fashion, by increasing the charge capacity control voltage in discrete steps towards the end of the integration period. Levine also teaches other methods of increasing the charge capacity control voltage and charge capacity towards the end of the integration period to extend the dynamic range of the imaging system, such as using enough multiple discrete steps to implement a continuously increasing charge capacity control voltage; or using linearly increasing charge capacity control voltage waveforms and increasing the slope or slopes of such waveforms.
However, although this method may be used to extend the dynamic range of a given imager, it is possible that the extended dynamic range may not be utilized for given frames. For example, a given scene may be relatively dark, thus wasting dynamic range at the expense of loss of contrast ratio and scene information content. This may be the case with, for example, imaging systems used for surveillance purposes.